As a catalyst carrier for cleaning exhaust gas of automobiles, a honeycomb-formed metal carrier comprising heat-resistant stainless steel is known. The metal carrier is formed by winding round a stack of a flat plate and a corrugated plate to form a honeycomb core body and joining the outer periphery of the honeycomb core body and inner surface of a metal outer cylinder.
Various methods are known as the production method of the metal carrier. For example, Japanese Patent Publication 63-44466 describes a metal carrier in which a pile of a flat plate and a corrugated plate is wound round, and the vicinity at least one of end faces of the wound honeycomb core body is soldered. However, such a metal carrier, when actually used on an internal combustion engine for automobile, tends to undergo deformation of the honeycomb core body or breakage of a foil on an outer periphery of the honeycomb core body due to thermal fatigue by heat cycle of heating and cooling. In particular, the exhaust gas inlet side of the honeycomb core body undergoes considerable cell deformation and foil breakage because it is exposed to high-temperature exhaust gas. Further, the metal carrier may be broken at the joined portion due to engine vibration and vibration during traveling.
Further, Japanese Patent Laid-open Publication 56-4373 discloses a honeycomb core body produced by a method in which solder is coated on a flat steel plate or at peaks of a corrugated steel plate through a supply roller from a vessel filled with liquid solder paste, and the former is wound round with a corrugated steel plate, or the latter is wound round with a flat steel plate. The patent also discloses a honeycomb core body produced by a method in which before coating solder, an adhesive is coated at a position to be solder coated through a supply device, and wound round while sprinkling a powder-formed solder material by a distribution device. In these honeycomb core bodies, the powder-formed solder material is piled in layers between the flat steel plate and the corrugated steel plate. Further, since the stack of the powder-formed solder material particles between the flat steel plate and the corrugated steel plate exists in amounts of twice the number of turns from the center of the honeycomb core body to the outermost periphery, when treated by vacuum soldering, the solder material melts to generate voids between the flat steel plate and the corrugated steel plate, and the voids in layers collect at several positions. Therefore, the honeycomb core body obtained by this method has unsoldered portions between the flat steel plate and the corrugated steel plate. Further, since the solder thickness increases at the joined portion of the flat steel plate and the corrugated plate, the core body tends to break at the brittle solder material and is insufficient in strength.
As another method, Japanese Patent Laid-open Publication 4-141238 discloses a production method of a metal carrier, in which a plurality of binder injectors having needles on the surface and backside of a corrugated foil are disposed in a single or multiple stages parallel to the corrugation, and operation of the binder injectors is selected so that the binder is coated at peaks of the corrugation in a previously designed range according to the movement of the corrugated foil to select joining positions of the corrugated foil and flat foil at desired positions of the honeycomb core body.
In producing a honeycomb core body by this method, to select a joining position of the corrugated foil and flat foil at a desired position of the honeycomb core body, first only the binder is supplied, after winding the honeycomb core body, or after containing it in an outer cylinder, a process to sprinkle the solder material from the end face of the honeycomb core body to apply the solder material to the binder coating portion. This results in an increase in cost. Further, when the binder is roll coated on the corrugated plate or the flat plate before forming the honeycomb core body and the solder material is also supplied, wound round into the honeycomb core body, and then soldering is made, since the solder material is sandwiched between the flat plate and corrugated plate, the solder material melts by soldering treatment to generate voids between both plates, and effective joining is not obtained. This is also true for a slurry which is a mixture of the binder and powder-formed solder. Since the binder and powder-formed solder are sandwiched between the flat plate and corrugated plate, the solder material melts after soldering to generate voids between both plates, and cannot provide effective joining. When the slurry is coated on the same joining position of the surface and backside of the flat plate and corrugated plate, voids further increase between both plates due to melting of the solder material from the slurry sandwiched between the flat plate and corrugated plate, resulting in insufficient joining. Further, Ni, Cr, and Si, which are ingredients of the solder material, diffuse into the base material, and formation of an intermetallic compound concentrates in a same position, degrading the strength.
Then, there have heretofore been known various types of exhaust gas cleaning apparatus, in which a metal honeycomb body is inserted into a metal outer cylinder, and a gas cleaning catalyst is carried on the metal honeycomb body. In particular, various types of honeycomb structures have been proposed to control or prevent peeling or damage of the joined portion due to a strain caused by a thermal stress generated by heat of exhaust gas of automobiles. For example, Japanese Patent Laid-open Publication 4-29750 discloses a metal carrier in which a metal honeycomb core body and an outer cylinder are joined with a plurality of bands formed at 1-15 mm ring-formed intervals in an axial direction of the metal carrier. This absorbs a stress generated in the axial direction of the carrier during cooling/heating cycle by deforming an unjoined portion at the ring-formed spacing to disperse a stress acting on the joined portion and unjoined portion. However, in some cases, depending on the joining condition of the metal honeycomb core body, the stress cannot be efficiently dispersed even by joining the plurality of bands between the outer cylinder and the metal honeycomb core body. For example, when the boundary of the metal honeycomb core body and the outer cylinder is in line with the boundary in the metal honeycomb core body, the stress is concentrated at that position due to the heat cycle, and the foil is broken from that position. Further, Japanese Patent Laid-open Publication 4-148016 discloses a metal carrier which has a region where a flat foil and a corrugated foil are joined in an axial direction in a length of 5 to 20% the axial direction length of the metal honeycomb core body. An outer layer reinforcing layer is joined in an axial direction within five layers from the outer most layer towards the inside, and the metal outer cylinder and the metal honeycomb core body are joined in a region in the axial direction within the outer layer reinforcing layer. In this case, the metal outer cylinder and the metal honeycomb core body are joined at a position in the axial direction to relax the thermal deformation in the axial direction of the metal outer cylinder and the metal honeycomb core body. However, because the metal outer cylinder and the metal honeycomb core body are joined only at a position, mechanical vibrations of the engine or vibrations of the vehicle during traveling tend to concentrate in the joined portion, leading to a breakage.
Further, Japanese Utility Model Laid-open Publication 2-83320 describes an exhaust gas cleaning apparatus comprising a plurality of metal honeycomb core bodies having a number of reticulated ventilation holes in an axial direction formed by stacking thin flat band materials and corrugated band materials to contact each other, mounted at intervals in a metal case with both ends open. Thus, an exhaust gas cleaning apparatus in which at least one position is joined is disclosed, except for a case that a contact surface of the outer peripheral surfaces of the individual metal honeycomb core bodies and the inner wall surface of the metal case are simultaneously joined with the inner wall surface of the metal case in the vicinity of both ends of the metal honeycomb core body. Japanese Utility Model Laid-open Publication 2-85814 describes an exhaust gas cleaning apparatus which comprises two metal honeycomb core bodies having a number of reticulated ventilation holes in the axial direction, manufactured by stacking thin metal flat band materials and corrugated band materials to contact each other, mounted with spacing in a cylindrical metal outer cylinder having both ends open and an enlarged diameter section, wherein the outer peripheral surface in the vicinity of the opening of the metal outer cylinder of each metal honeycomb core body is mounted to the inner wall surface of the metal outer cylinder, and the outer peripheral surface in the vicinity of the spacing of the individual metal honeycomb core bodies does not contact the inner wall surface of the metal outer cylinder.
In these exhaust gas cleaning apparatus, a plurality of metal honeycomb core bodies are disposed at spaces in a metal outer cylinder, and only a portion in the axial direction of the contact surface of the outer peripheral surface of the individual metal honeycomb core body and the inner wall surface of the metal outer cylinder is annularly joined, preferably over the entire periphery to disperse and relax thermal stress, thereby preventing peeling of the outer peripheral surface of the metal honeycomb core body and the inner wall surface of the metal outer cylinder or peeling or cracking of the flat plates and the corrugated plates forming the metal honeycomb core bodies due to thermal stress and thermal deformation.
However, even when thermal stress is dispersed and relaxed by the above method, it is impossible to absorb a strain in the vicinity of the outer peripheral surface of the metal honeycomb core body due to thermal stress and thermal deformation generated in the exhaust gas cleaning apparatus. Therefore, the vicinity of the peripheral surface is liable to be subjected to thermal stress and thermal deformation, resulting in cell deformation or peeling of the flat plate and corrugated plate. Further, joining the inner wall surface of the metal outer cylinder and the outer peripheral surface of the metal honeycomb core body at only a portion tends to weaken the joining, and cannot withstand the pressure of exhaust gas flow at high temperatures.
Further, in a conventional metal carrier, a plurality of metal honeycomb core bodies are joined at spaces in a metal outer cylinder (e.g. Japanese Utility Model Laid-open Publication 2-83320). In this carrier, in an attempt to remove thermal stress and thermal deformation generated in the metal carrier at high temperatures by only joining of the outer peripheral surface of the metal honeycomb core body and the inner wall surface of the metal outer cylinder, a strain occurs that cannot be removed, and the metal honeycomb core body may be broken by the strain if the metal honeycomb core bodies are not joined at spaces. This easily leads to a large-sized exhaust gas cleaning apparatus, except for a case to intentionally generate a turbulent flow. Further, as described in Japanese Utility Model Laid-open Publication 2-85815, a large-diameter section provided in the metal outer cylinder leads to a large-sized exhaust gas cleaning apparatus.
On the other hand, in an exhaust gas cleaning apparatus carrying a gas cleaning catalyst on a metals honeycomb core body, it has been known to use a curved metal carrier. For example, Japanese Utility Model Laid-open Publication 4-78938 proposes a curved metal carrier, in which flat plates and corrugated plates are stacked in alternation to form a honeycomb structure, and the honeycomb structure is inserted into an outer cylinder. In the metal carrier, arrays of slits is provided so that each slit has a length of more than one wave in the cross direction of the flat plates and corrugated plates, the slits are arranged at spaces in the longitudinal direction, and a number of the arrays are formed in parallel and shifted front each other.
However, in the curved metal carrier, since the flat plates and corrugated plates forming the metal honeycomb core body are provided with slits and inserted into a curved metal outer cylinder, if thermal stress and thermal deformation occur in the metal carrier at high temperatures, the slits expand to generate cracking, cell deformation, peeling of joined portions, and breakage of the metal honeycomb core body. Further, since the flat plates and corrugated plates have slits, when the metal honeycomb core body is inserted into the curved metal outer cylinder after the carrier is formed, the honeycomb structure cannot be maintained, and the predetermined cell structure cannot be maintained. That is, when designing a metal carrier, it is difficult to ensure the desired porosity ratio and surface area. Further, due to the slits, exhaust gas tends to flow only the vicinity of the outer peripheral surface of the curved metal outer cylinder, resulting in a considerably deteriorated efficiency of exhaust gas cleaning.
Then, the inventors have conducted intensive studies to solve such defects and found that flat plates and corrugated plates are soldered with a solder material coated in spots or stripes on the peaks of the corrugated plate to form a metal honeycomb core body, while the solder material coated on the metal honeycomb core body still has a fluidity and the solder material coated on the inner wall surface of the metal outer cylinder has no fluidity, the outer peripheral surface of the metal honeycomb core body and the inner wall surface of the metal outer cylinder are assembled, and then soldering heat treated, thereby sufficiently absorbing a thermal stress and achieving the present invention. Therefore, a primary object of the present invention is to provide a stable metal carrier which sufficiently absorbs a thermal stress and, when used on vehicles, can withstand practical use for an extended period of time.